efi/libstub: Make initrd file loader configurable

Loading an initrd passed via the kernel command line is deprecated: it
is limited to files that reside in the same volume as the one the kernel
itself was loaded from, and we have more flexible ways to achieve the
same. So make it configurable so new architectures can decide not to
enable it.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>